Bridge circuits are used in a wide range of applications. A typical 3-phase bridge circuit for a motor drive is shown in FIG. 1. Each of the three half bridges 15, 25, 35 in circuit 10 includes two switches (61-66), which are able to block current in one direction and are capable of conducting current in both directions. Because the transistors (41-46) commonly used in power circuits are inherently incapable of conducting current in the reverse direction, each of the switches 61-66 in circuit 10 comprises a transistor (41-46) connected anti-parallel to a freewheeling diode 51-56. The transistors 41-46 are each capable of blocking a voltage at least as large as the high voltage (HV) source of the circuit 10 when they are biased in the OFF state, and diodes 51-56 are each capable of blocking a voltage at least as large as the high voltage (HV) source of the circuit 10 when they are reverse biased. Ideally, the diodes 51-56 have good switching characteristics to minimize transient currents during switching, therefore Schottky diodes are commonly used. The transistors 41-46 may be enhancement mode (normally off, Vth>0), i.e., E-mode, or depletion mode (normally on, Vth<0), i.e., D-mode devices. In power circuits enhancement mode devices are typically used to prevent accidental turn on in order to avoid damage to the devices or other circuit components. Nodes 17, 18, and 19 are all coupled to one another via inductive loads, i.e., inductive components such as motor coils (not shown in FIG. 1).
FIG. 2a shows half bridge 15 of the full 3-phase motor drive in FIG. 1, along with the winding of the motor (inductive component 21) between nodes 17 and 18 and the switch 64 which the motor current feeds into. For this phase of power, transistor 44 is continuously on (Vgs44>Vth) and transistor 42 is continuously off (Vgs42<Vth, i.e., Vgs42=0V if enhancement mode transistors are used), while transistor 41 is modulated with a pulse width modulation (PWM) signal to achieve the desired motor current. FIG. 2b, which is a simplified version of the diagram in FIG. 2a, indicates the path of the current 27 during the time that transistor 41 is biased on. For this bias, the motor current flows through transistors 41 and 44, while no current flows through switch 62 because transistor 42 is biased off and diode 52 is reverse biased. Referring to FIG. 2c, during the time that transistor 41 is biased off, no current can flow through transistor 41 or diode 51, and so the motor current flows through diode 52. During this portion of operation, the inductive component 21 forces the voltage at node 17 to a sufficiently negative value to cause diode 52 to conduct.
Currently, insulated gate bipolar transistors (IGBTs) are typically used in high power bridge circuits, and silicon MOS transistors, also known as MOSFETs, are used in low power applications. Traditional IGBTs inherently conduct in only one direction, and so a freewheeling diode is required for proper operation of a switch with an IGBT. A standard MOS transistor inherently contains an anti-parallel parasitic diode. As seen in FIG. 3a, if the gate and source of a MOS device 50 are biased at the same voltage and the drain is biased at a lower voltage, such as occurs in transistor 42 when transistor 41 is off (FIG. 2c), parasitic diode 60 prevents the intrinsic MOS transistor 71 from turning on. Therefore, the path of the reverse current 37 is through the parasitic diode 60. Because the parasitic diode 60 inherently has poor switching characteristics, the parasitic diode 60 experiences large transients when MOS device 50 is switched on or off.
To completely prevent turn on of the parasitic diode 60, the 3-component solution illustrated in FIG. 3b is often employed. In FIG. 3b, diode 69 is added to the switch to prevent any current from flowing through the parasitic diode 60, and a Schottky diode 68 is added to carry the current during the time that current flows in the direction shown in FIG. 3b, i.e., from the source side to the drain side of MOS device 50.